Sample-size planning is currently the Achilles heel of biomedical research. Many fields of preclinical research routinely rely on inadequate samples, leading to waste, excessive false-positives, and inflated effect sizes. These practices persist because researchers often eschew sound sample-size planning for easy but invalid approaches. To help address this critical issue, this project will produce a self-guided online course that will help trainees exchange poor sample-size determination practices for sound approaches. The course will a) help redirect researchers away from erroneous approaches by making clear their severe negative consequences, b) will dispel common misconceptions that discourage sound sample-size planning, and c) will provide exercises with instant feedback to build self-efficacy for independent sample-size planning. The materials will be produced for a modest budget, will incorporate guidance from a team of renowned methodological experts, and will be anchored in a proven behavioral-change approach. The course will provide a needed supplement to current materials, which do not adequately teach sample-size determination. A thoughtful piloting plan will enable extensive feedback from diverse stakeholders to ensure strong impact not only on knowledge but also on attitudes and actual competency. Dissemination will be supported through partnerships with professional societies and the extensive social media reach of the participating guest lecturers. Funding this proposal can have a strong positive impact on rigor and reproducibility in preclinical research.